1. Field of the Invention
The present invention relates to an internal voltage fall-down circuit of a semiconductor device. In particular, the present invention relates to an internal voltage fall-down circuit which can test fuse programs for controlling an internal power supply voltage by pad signals without fuse blowing.
2. Description of the Prior Art
A conventional internal voltage fall-down circuit includes a reference voltage generating section 10, a reference voltage transforming section 20 and a driver section 30, as shown in FIG. 1, wherein an output signal Vint from the conventional internal voltage fall-down circuit is input to an internal circuit 40 as supply voltage. An output signal VR2 from the reference voltage generating section 10 is input to a first input terminal of a first comparator 21 in the reference voltage transforming section 20, and an output signal VR from the reference voltage transforming section 20 is used as a final comparison voltage of the driver section 30.
A reference voltage generator 11 in the reference voltage generating section 10 outputs a stabilized voltage VR1 regardless of external voltage fluctuations. Common types of it are a bandgap reference voltage generator or Windler current source. The output voltage VR1 from the reference voltage generator 11 is input to the first input terminal of the first comparator 12 in the voltage amplifier 16. Then an output voltage VR2 from the reference voltage generator 16 is divided into a given voltage Va by a voltage divider consisted of fixed resistors 14, 15, which is then input to the second input terminal of the first comparator 12. A fallen reference voltage VR2 is then output from a first current driver 13 connected to the output terminal of the first comparator 12.
The resistor 15 is a fixed resistor to provide a single resistance value corresponding to fuse programs.
The reference voltage transforming section 20 performs a normal mode and a stress mode operation and then outputs an output voltage in a normal mode operation, wherein the reference voltage VR2 from the reference voltage generating section 10 is input to the first input terminal of a second comparator 21 used in a normal mode operation, the output voltage VR is feedbacked to the second input terminal of the second comparator 21 thereof, and the second current driver 22 is connected to the output terminal of the second comparator 21 thereof.
The reference voltage transforming section 20 outputs the output voltage VR in a stress mode operation, wherein a bias voltage VST from a bias circuit 23 is input to the first input terminal of a third comparator 24 used in a stress mode operation, the output voltage VR is feedbacked to the second input terminal of the third comparator 24 thereof and a third current driver 25 is connected to the output terminal of the third comparator 24 thereof.
Here, the term "a normal mode operation" means that "supply voltage=3.3V.+-.10% and the term "a stress mode operation" means that "supply voltage is more than 1.5.times.3.3V".
In addition, in a normal mode operation, since the second current driver 22 is enabled by the second comparator 21 and the third current driver 25 is enabled by the third comparator 24, the resulting output voltage VR holds the reference voltage VR2 from the reference voltage generating section 10. In a stress mode operation, since the second current driver 22 is enabled by the second comparator 21 and the third current driver 25 is enabled by the third comparator 24, the resulting output voltage VR holds the bias voltage VST from the bias circuit 23. Meanwhile, as the node onto which the bias voltage will be carried is connected to the bias circuit 23 and the fall-down current sink 27, the bias voltage VST keeps "supply voltage-nVt(n=2)".
The driver section 30 is used to provide current corresponding to each state of operation in the internal circuit 40. However, when the supply voltage is turned on, the driver section 30 may be consisted of standby drivers 31, 32 and 35, and activation drivers 33, 34 which are activated by an enable clock ACT only during an active mode. The standby drivers 31, 32 and 35 has a structure of voltage follower type, in which the fall-down current sink 35 is connected to the node for outputting the internal supply voltage Vint from the internal circuit 40 and a ground voltage terminal. The activation drivers 33, 34 are also voltage follower types.
The internal circuit 40 may be an on-chip circuit which employs the internal supply voltage Vint, a given value of which is fallen down, from an external supply voltage.
Normally, in the above-mentioned internal voltage fall-down circuit, variations in processes or noises occurring during operation of the on-chip circuit may cause the internal supply voltage levels to fluctuate. Accordingly, in order to compensate for the fluctuations in the internal supply voltage level, it is preferred that the above reference voltage VR2 is controlled using a fuse program, when the reference voltage of the comparator for driving the final current driver.
Here, the variations in processes mean threshold voltage Vt or saturation current Ids. The noises occurring during operation of the on-chip circuit mean current spikes which cause a large current flow at a sensing or an input/output circuit, noise of which affects the internal circuit to cause change of preset voltage (i.e., change in potentials of the reference voltage).
Accordingly, the above-mentioned conventional internal voltage fall-down circuit has problems that it could compensate for the level changes in or test the reference voltage VR2 from the reference voltage generating section 10, and could measure information for fuse blowing, only after programming of the fuses built in the resistor 15 of the reference voltage generating section 10 is performed.